These "spikes" you speak of are cpu related and have nothing to do with the temperature of the fluid. The readings you see in monitoring software are the actual cpu temps...not the fluid. Fluids in the pc loop can't and don't react as quickly as what you see recorded in monitoring software. Put a temp probe in any reservoir and you will quickly understand. The volume of fluid does absolutely nothing other than a greater volume takes a bit longer to equalize.
Take out the better pumps/blocks/radiator from your above statement and simply add more fluid to a AIO and it is going to perform exactly the same as it did before the extra fluid was introduced...with the exception it will take slightly longer to reach the original temperatures it did initially. Your ability/capacity to cool said fluid has not changed...a larger reservoir is certainly not going to provide this. The only meaningful reservoir is a radiator. Reservoirs are only a "thing" because they are a convenient way to fill/drain a system and add some "bling" to the loop.
You...are missing the point of the comment here. Let me put this into automotive terms for you.
Imagine a car with a 10 gallon radiator tank. Now imagine that you drive the car in a number or ways.
In situation 1 you drive in town, and have immense spikes because between each light you slam on the gas. Very large transient spikes.
In situation 2 you are driving on a highway instead of town, you don't have as many spikes, but so you get up to speed a little slower but remain there longer. That said, you still have stops.
In situation 3 you are on a freeway. There are slight peaks to pass, but otherwise you run at a constant speed.
A large capacity radiator does nothing for situation 1. As you've said, the peaks are so transient that they aren't considered.
Situation 2 is perfect for that high capacity reservoir. You have consistent energy dumps, but not constant enough to heat everything up.
Situation 3 renders the high capacity reservoir useless again. You have enough energy to heat all the fluid up, so it's not doing you any good.
This matters because most gaming is going to fall into situation 2. It's not a render farm or server (situation 3). It's not a casual gaming session or web browser (situation 1). It's a number of peaks and input, that doesn't max out the thermal load. This is how you have people that can run much quieter systems because the fans are at 20-40% instead of 80-100%. They don't require the constant dump, have enough capacity to absorb the load peaks without having to spin up, but also can reject fast enough under low loading to be fine. Ironically this is what people often miss about AIOs...as with basically no fluid they have to dynamically respond to loads to prevent issues. Custom loops on the other hand can be tailored to usage...and depending upon yours a larger reservoir could be a good idea.
Side note, the delta in temperature is why cars have overflow reservoirs. Fire is indeed hot, unlike the few degrees delta present in a computer loop....given the low delta between CPU and ambient.
Side note on the copper, good preparation is always key to minimizing the solder. Someone made me aware of an acid treatment instead of the traditional preparation of the surface. You might want to pursue that, as I've seen it make a joint where all of the metal actually flowed into the joint and was nearly imperceptible. I was a skeptic, until I had a chance to pressurize it. 60% of the strength of the pipe before it blew...needless to say I am a believer. I'm more use to 40% or less.
